cDNA cloning of an aryl hydrocarbon receptor from Baikal seals (Phoca sibirica)

Gene cloning and expression analysis of AhR and CYP4 from Pinctada martensii after exposed to pyrene

Pyrene, a typical polycyclic aromatic hydrocarbon, is a common pollutant in the marine environment. Polycyclic aromatic hydrocarbons initiate cellular detoxification in an exposed organism via the activation of the aryl hydrocarbon receptor (AhR). Subsequent metabolism of these xenobiotics is mainly by the cytochrome P450 enzymes of the phase I detoxification system. Full-length complementary DNA sequences from the pearl oyster Pinctada martensii (pm) encoding AhR and cytochrome P4 were cloned. The P. martensii AhR complementary DNA sequence constitutes an open reading frame that encodes for 848 amino acids. Sequence analysis indicated PmAhR showed high similarity with its homologues of other bivalve species. The cytochrome P(CYP)4 complementary DNA sequence of P. martensii constitutes an open reading frame that encodes for 489 amino acids. Quantitative real-time analysis detected both PmAhR and PmCYP4 messenger RNA expressions in the mantle, gill, hepatapancreas and adductor muscle of P. martensii exposed to pyrene. The highest transcript-band intensities of PmAhR and PmCYP4 were observed in the gill. Temporal expression of PmAhR and PmCYP4 messenger RNAs induction was observed in gills and increased between 3 and 5 days post exposure; then returned to control level. These results suggest that messenger RNAs of PmAhR and PmCYP4 in pearl oysters might be useful parameters for monitoring marine environment pyrene pollution.

Enzymatic characterization of in vitro-expressed Baikal seal cytochrome P450 (CYP) 1A1, 1A2, and 1B1: implication of low metabolic potential of CYP1A2 uniquely evolved in aquatic mammals

This study aimed to elucidate the catalytic function of cytochrome P450 (CYP) 1 enzymes in aquatic mammals. Alkoxyresorufin O-dealkylation (AROD) activities including methoxy- (MROD), ethoxy- (EROD), pentoxy- (PROD), and benzyloxyresorufin O-dealkylation (BROD), and 2- and 4-hydroxylation activities of 17β-estradiol (E2) were measured by using yeast-expressed Baikal seal (Pusa sibirica) CYP1A1, 1A2, and 1B1 proteins. Heterologous protein expression of the Baikal seal CYP1s (bsCYP1s) in yeast microsomes was confirmed by reduced CO-difference spectra and immunoblotting. Heterologously expressed human CYP1 enzyme (hCYP1) activities were simultaneously measured and compared with those of bsCYP1 isozymes. Recombinant bsCYP1A1 protein showed the highest Vmax of EROD, followed by MROD, PROD, and BROD, similar to that of hCYP1A1. Vmax/Km ratios of all AROD activities catalyzed by bsCYP1A1 were lower than those catalyzed by hCYP1A1, suggesting less potential for AROD by bsCYP1A1. Enzymatic assays for bsCYP1A2 showed no or minimal AROD activities, while hCYP1A2 displayed MROD and EROD activities. bsCYP1B1 showed an AROD profile (EROD>BROD>MROD>>PROD) similar to that of hCYP1B1; however, Vmax/Km ratios of all AROD activities by bsCYP1B1 were higher. Yeast microsomes containing bsCYP1A1 and 1B1 and hCYP1A1, 1A2, and 1B1 metabolized E2 to 2-OHE2 and 4-OHE2, whereas bsCYP1A2 showed no such activity. Comparison of 4- and 2-hydroxylations of E2 by CYP1As suggests that bsCYP1A1, hCYP1A1, and 1A2 preferentially catalyze 2- rather than 4-hydroxylation. As for CYP1B1, the Vmax/Km ratios suggest that both Baikal seal and human CYPs catalyze 4- rather than 2-hydroxylation. Interspecies comparison showed that bsCYP1B1 has higher metabolic potencies for both E2 hydroxylations than does hCYP1B1, whereas the activity of bsCYP1A1 was lower than that of hCYP1A1. Messenger RNA expression levels of bsCYP1s in the liver of Baikal seals indicated that bsCYP1A1 and 1A2 enzymes contributed to 16.2% and 83.7% of total CYP1s, respectively; bsCYP1B1 accounted for only 0.06%. Addition of anti-human CYP1A1 antibody in seal liver microsomes suppressed EROD activity more than did anti-human CYP1A2 antibody. Therefore, EROD may be catalyzed by hepatic bsCYP1A1 but not bsCYP1A2, consistent with the results of yeast-expressed bsCYP1A1 and 1A2. In silico substrate-docking models of bsCYP1s suggested that the defect in bsCYP1A2 enzymatic activities may be accounted for by the Pro substitution of highly conserved Thr in the I-helix, which is involved in formation of a hydrogen bond with the hydroperoxy intermediate on the heme. This Thr-Pro substitution is evolutionarily conserved across aquatic mammals and could explain their lower metabolic potential for persistent organic pollutants.

Immune-relevant and new xenobiotic molecular biomarkers to assess anthropogenic stress in seals

Harbour seals as top predators and indicators for ecosystem health are exposed to increasing pressure caused by anthropogenic activities in their marine environment. After their lactation period of about 24 days pups are weaned and left to hunt on their own. Little is known about the development of their immune system and a better understanding of anthropogenic impacts on the general health and immune system of harbour seal pups is needed. mRNA transcription of six immuno-relevant biomarkers was analysed in 13 abandoned harbour seal pups from the North Sea, fostered at the Seal Centre Friedrichskoog, Germany. RNAlater blood samples were taken at admission, day 22 and before release and analysed using RT-qPCR. Significant differences in HSP70, cytokine IL-2 and xenobiotic biomarkers AHR, ARNT and PPARα transcription were found between admission, during rehabilitation and before release. Highest levels at admission may result from dehydration, handling, transport and contaminant exposure via lactation. The significant decrease is linked to health improvement, feeding and adaptation. The increase before release is suspected to be due to infection pressure and contaminant exposure from feeding on fish. Molecular biomarkers are a sensitive tool to evaluate health and pollutant exposure and useful to serve as early warning indicators, monitoring and case-by-case tool for marine mammals in human care and the wild.

Persistent organic pollutants in the endangered Hawaiian monk seal (Monachus schauinslandi) from the main Hawaiian Islands

Little is known about levels or effects of persistent organic pollutants (POPs) in Hawaiian monk seals (HMS) from the main Hawaiian Islands (MHI) subpopulation. This study examined concentrations of a large suite of POPs in blubber and serum of juvenile and adult HMS from the MHI. Adult females have the lowest blubber levels of most POPs, whereas adult males have highest levels. POPs in serum were significantly different in adult males compared with adult females for chlordanes and summed dichlorodiphenyltrichloroethanes (DDTs). Lipid-normalized concentrations of chlordanes, DDTs, polychlorinated biphenyls, and mirex in paired blubber and serum samples were significantly correlated. Contaminant levels from the MHI were at similar or lower levels than those from remote Northwestern Hawaiian Island populations. Determining initial ranges of POPs is an important step towards assessing one of the many potential health threats to this critically endangered species.

Integrative assessment of potential effects of dioxins and related compounds in wild Baikal seals (Pusa sibirica): application of microarray and biochemical analyses

We have previously indicated that accumulation of chlorinated dioxins and related compounds (DRCs) induced cytochrome P450 (CYP) 1A1, 1A2 and 1B1 isozymes in the liver of wild Baikal seals (Pusa sibirica). Here we attempt to assess the potential effects of DRCs triggered by the induction of these CYP1 isozymes in this species, using an integrative approach, combining gene expression monitoring and biochemical assays. To screen genes that may potentially respond to the exposure of DRCs, we constructed a custom cDNA oligo array that can target mRNAs in Baikal seals, and monitored hepatic mRNA expression levels in the wild population. Correlation analyses between the hepatic total 2,3,7,8-tetrachlorodibenzo-p-dioxin toxic equivalents (TEQs) and mRNA levels supported our previous findings that high accumulation of DRCs induces the transcription of CYP1A1, CYP1A2 and CYP1B1 genes. In addition, our integrative assessment indicated that the chronic exposure to DRCs may alter the hepatic transcript levels of genes related to oxidative stress, Fe ion homeostasis, and inflammatory responses. The expression levels of CYP1A2 showed significant positive correlations with levels of malondialdehyde, a biomarker of lipid peroxidation, and of etheno-dA, a DNA adduct, suggesting that the lipid peroxidation may be enhanced through the production of reactive oxygen species (ROS) triggered by CYP1A2 induction. Moreover, there was a positive correlation between heme oxygenase activities and malondialdehyde levels, suggesting the prompted heme degradation by ROS. Fetuin-A levels, which are suppressed by inflammation, showed a significant negative correlation with TEQ levels, and hepcidin levels, which are conversely increased by inflammation, had significant positive correlations with malondialdehyde and etheno-dA levels, implying the progression of inflammation by DRC-induced oxidative stress. Taken together, we propose here that wild Baikal seals may suffer from effects of chronic exposure to DRCs on the induction of CYP1 isozymes, followed by increased oxidative stress, heme degradation and inflammation.

Evaluation of relative potencies for in vitro transactivation of the baikal seal aryl hydrocarbon receptor by dioxin-like compounds

To evaluate the sensitivity and responses to dioxins and related compounds (DRCs) via aryl hydrocarbon receptor (AHR) in Baikal seals (Pusa sibirica), we constructed an in vitro reporter gene assay system. Baikal seal AHR (BS AHR) expression plasmid and a reporter plasmid containing CYP1A1 promoter were transfected in COS-7 cells. The cells were treated with six representative congeners, and dose-dependent responses were obtained for all the congeners. EC50 values of 2,3,7,8-TCDD, 1,2,3,7,8-PeCDD, 2,3,7,8-TCDF, 2,3,4,7,8-PeCDF, and PCB126 were found to be 0.021, 1.8, 0.16, 2.4, and 2.5 nM, respectively. As the response did not reach the maximal plateau, EC50 value for PCB118 could not be obtained. The TCDD-EC50 for BS AHR was as high as that for dioxin sensitive C57BL/6 mouse AHR. The in vitro dose responses were further analyzed following an established systematic framework and multiple (20, 50, and 80%) relative potencies (REPs) to the maximum TCDD response. The estimates revealed lower REP ranges (20-80%) of PeCDD and PeCDF for BS AHR than for mouse AHR. Average of the 20, 50, and 80% REPs was designated as Baikal seal specific TCDD induction equivalency factor (BS IEF). The BS IEFs of PeCDD, TCDF, PeCDF, PCB126, and PCB118 were estimated as 0.010, 0.018, 0.0078, 0.0059, and 0.00010, respectively. Total TCDD induction equivalents (IEQs) that were calculated using BS IEFs and hepatic concentrations in wild Baikal seals corresponded to only 12-31% of 2005 WHO TEF-derived TEQs. Nevertheless, about 50% of Baikal seals accumulated IEQs over the TCDD-EC50 obtained in this study. This assessment was supported by the enhanced CYP1A1 mRNA expression found in 50% of the specimens contaminated over the TCDD-EC50. These findings suggest that the IEFs proposed from this in vitro assay could be used to predict AHR-mediated responses in wild seals.

Accumulation features and temporal trends of PCDDs, PCDFs and PCBs in Baikal seals (Pusa sibirica)

This study investigated the accumulation features and temporal trends of PCDD/Fs, dioxin-like PCBs (DL-PCBs) and non-dioxin-like PCBs (NDL-PCBs) in the blubber of Baikal seals collected in 1992 and 2005. DL-PCBs (480-3600ng/g) and NDL-PCBs (980-35,000ng/g) were dominant contaminants. Concentrations of PCDDs and PCBs in males were significantly higher than in females. In males, age-dependent accumulation was observed for PCDDs, mono-ortho PCBs and NDL-PCBs. PCDFs and non-ortho PCBs showed no such trends, implying that exposure of seals to these contaminants has been decreasing in recent years. No decreasing temporal trend was observed for PCDDs, mono-ortho PCBs and NDL-PCBs, suggesting that Baikal seals are still exposed to PCDDs and PCBs. TEQs of PCDDs and mono-ortho PCBs in seals collected in 2005 accounted for 62-77% of total TEQs. The TEQ levels in 40% of the specimens exceeded the threshold level for immunosuppression observed in harbor seals (209pg/g).

Contamination and effects of perfluorochemicals in Baikal seal (Pusa sibirica). 2. Molecular characterization, expression level, and transcriptional activation of peroxisome proliferator-activated receptor alpha

To investigate the biological effects of perfluorochemicals (PFCs) and to identify biomarkers of exposure to PFCs, this study focused on the effects mediated by peroxisome proliferator-activated receptor alpha (PPARalpha) in Baikal seals (Pusa sibirica). We cloned a full-length cDNA, encoding PPARalpha from the liver of Baikal seal, which has a deduced open reading frame of 468-amino acid residues with a predicted molecular mass of 52.2 kDa. Comparison of the amino-acid sequence of Baikal seal PPARalpha with that of other mammalian PPARalpha showed considerable similarities with PPARalpha of dog (97%), human (95%), rat (92%), and mouse (91%). The quantitative real-time RT-PCR analyses of tissues from Baikal seals revealed that PPARalpha mRNAs were primarily expressed in the liver, kidney, heart, and muscle. The hepatic expression levels of PPARalpha mRNA showed a positive correlation with the expression levels of immunochemically detected cytochrome P450 (CYP) 4A-like protein, indicating that the PPARalpha-CYP4A signaling pathway in Baikal seal is likely conserved. This study also developed an in vitro PPARalpha reporter gene assay using African green monkey kidney CV-1 cells transiently transfected with Baikal seal PPARalpha cDNA expression vector and a reporter vector containing a peroxisome proliferator-responsive element The in vitro reporter gene assay displayed significant response to clofibrate, which is a known PPARalpha agonist in humans and rodents. Treatmentwith perfluorooctanoic acid (PFOA), perfluorononanoic acid (PFNA), perfluorodecanoic acid (PFDA), perfluoroundecanoic acid (PFUnDA), or perfluorooctane sulfonate (PFOS) induced PPARalpha-mediated transcriptional activity in a dose-dependent manner, showing the lowest-observed-effect concentrations of 62.5, 125, 125, 62.5, and 125 microM, respectively. In the livers of wild Baikal seals, expression levels of PPARalpha mRNA showed a significant positive correlation with PFNA levels. Moreover, expression of hepatic CYP4A-like protein was significantly correlated with the hepatic concentrations of PFNA and PFDA. These results suggest modulation of the PPARalpha-CYP4A signaling pathway by PFCs in the wild Baikal seals. Our study demonstrates that the PPARalpha-mediated response may be a useful biomarkerto evaluate potential biological effects of PFCs in wildlife.

Aryl hydrocarbon receptor (AHR) and AHR nuclear translocator (ARNT) expression in Baikal seal (Pusa sibirica) and association with 2,3,7,8-TCDD toxic equivalents and CYP1 expression levels

Most toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related planar halogenated aromatic hydrocarbons (PHAHs) are mediated by ligand-activated aryl hydrocarbon receptor (AHR) signaling pathway. To understand the regulation mechanism of AHR and AHR nuclear translocator (ARNT) expression in wild Baikal seal (Pusa sibirica) population contaminated by PHAHs, the present study investigated hepatic mRNA expression levels of AHR and its heterodimer, ARNT genes, in association with biological index (age, gender and body weight), PHAH accumulation and expression levels of cytochrome P450 (CYP) 1A and 1B. While there was no gender difference, the AHR mRNA expression levels were increased with ages (p = 0.014) and body weights (p = 0.015), indicating that AHR expression might be affected by these biological factors. The AHR mRNA expression levels exhibited significant positive correlations with total TEQs and most of individual congener TEQs derived from polychorinated dibenzo-p-dioxins, dibenzofurans and non-ortho coplanar polychorinated biphenyls (PCBs), indicating the transcriptional up-regulation of AHR expression by these congeners. On the other hand, there was no significant correlation between individual TEQs from mono-ortho coplanar PCBs and AHR expression. These results imply the structure-related transcriptional activity of AHR among PHAHs congeners. AHR mRNA levels showed positive correlations with both CYP1A protein (p = 0.039) and CYP1A1 mRNA expression levels (p = 0.046). In contrast to AHR expression, neither the total nor individual congener TEQs influenced ARNT at the transcriptional level. ARNT mRNA showed significant negative correlations with CYP1A/1B protein (p = 0.027 and p = 0.006) and CYP1A1 mRNA expression levels (p = 0.039), implying the existence of different transcriptional regulation between AHR and ARNT genes and negative regulation by CYP1A/1B-mediated signaling pathways. The present findings may render significant insight on the basic mechanisms underlying regulation of AHR and ARNT expressions associated with biological factors and PHAH exposure in wild mammalian populations.

Aryl hydrocarbon receptors: diversity and evolution

Animals have evolved inducible enzymatic defenses to facilitate the biotransformation and elimination of toxic compounds encountered in the environment. The sensory component of this system consists of soluble receptors that regulate the expression of certain isoforms of cytochrome P450, other enzymes, and transporters in response to environmental chemicals. These receptors include several members of the steroid/nuclear receptor superfamily as well as the aryl hydrocarbon receptor (AHR), a member of the bHLH-PAS gene superfamily. In addition to its adaptive functions, the AHR serves poorly understood physiological roles; interference with those roles by dioxins and related chemicals causes toxicity. One approach to understanding the physiological significance of the AHR is to characterize its structure, function, and regulation in diverse species, including mammals, birds, fish, and invertebrates. These animal groups include model species with unique features that can be exploited to broaden our understanding of AHR function. Studies carried out in diverse species also provide phylogenetic information that allows inferences about the evolutionary history of the AHR. This review summarizes the current understanding of AHR diversity among animal species and the evolution of the AHR signaling pathway, as inferred from molecular studies in vertebrate and invertebrate animals. The AHR gene has undergone duplication and diversification in vertebrate animals, resulting in at least three members of an AHR gene family: AHR1, AHR2, and AHR repressor. The inability of invertebrate AHR homologs to bind dioxins and related chemicals, along with other evidence, suggests that the adaptive role of the AHR as a regulator of xenobiotic metabolizing enzymes may have been a vertebrate innovation. The physiological functions of the AHR during development appear to be ancestral to the adaptive functions. Sensitivity to the developmental toxicity of dioxins and related chemicals may have had its origin in the evolution of dioxin-binding capacity of the AHR in the vertebrate lineage.